According to the Washington Post the IRS is considering taxing virtual goods in 2009:
"The nation's taxpayer advocate, who recommends to the IRS how to improve the average consumer's tax-paying process, released her annual report Wednesday describing some of the most serious problems encountered by taxpayers as well as some issues that the IRS should proactively address.
...But she also told the agency that it should "proactively address emerging issues such as those arising from virtual worlds." Her report said that about $1 billion in real dollars changed hands in computer-based environments during 2005. Additionally, more than 16 million people are said to have active subscriptions in these worlds, "many of which have their own virtual economies and currencies."
...She suggests that to improve voluntary tax compliance, the IRS issue guidance addressing how taxpayers should report economic activities in virtual worlds."
This follows the recent trends such as the New York State Tax in other countries such as China or Sweden.
If the U.S government taxes RMT activities it gives it a great deal of legal legitimacy (depending on what and how they tax) and folows an international trend.